Research Spotlight

Two University astronomers, Kris Davidson and Roberta Humphreys have discovered that a massive binary star system, Eta Car is returning to its original state: that of a hot star easily visible in the sky of the Southern Hemisphere. Davidson and Humphreys have observed Eta Car since the Hubble Space Telescope first came online in 1990. This series of observations has given them solid evidence that Eta Car-- sometimes called the poster child for very massive stars, because it is the most massive star that is close enough to Earth to be easily observed—is experiencing dramatic changes.

About 170 years ago, Eta Car, underwent a great eruption, an event so big, it could have been mistaken for a supernova. While the energy released in this eruption was similar to a supernova, there were key differences. This “imposter” event did not destroy Eta Car, unlike a real super nova and no new elements were synthesized in the event, which is another hallmark of an authentic super nova.

Davidson says that astrophysicists do not understand what causes imposter eruptions. “It may be some kind of instability in the outer layers that probably effects all very massive stars, but that does not explain why some have impostor events and others do not.”

About ten years ago, Davidson and Humphreys were looking at Eta Car with the Hubble telescope, and in the course of that project discovered that it was changing rapidly. In a matter of a few years the star had changed so substantially that it looked like a different object . “Stars are not supposed to do that. I think it’s probably recovering from its great eruption. It’s bizarre that the recovery is not steady. It goes in fits and starts,” Davidson said. During that period the star got four times as bright as it had been and became visible with the naked eye again. “The simplest explanation for the change is a decrease in the outflow of mass from the star in the form of a stellar wind 1000 times as strong as in other massive stars.” Davidson says that In terms of its luminosity Eta Car is one in a billion, like finding an eight foot tall person. Recently, it appears that the mass flowing out of it is decreasing, in a way that has not been seen in more than a hundred years. Eta Car now looks similar to the way that it did when Edmund Halley observed it in the 19th century. It may return to this previous state in the next ten years or so, if it continues to lose mass at this rate.

Davidson says the Hubble is 10 times as good as ground-based telescopes for making these observations. The Hubble can measure ultraviolet light which ground based telescopes cannot. There were some very dense knots of gas thrown out of Eta Car about 100 years ago. When ground based telescopes observe Eta Car everything is seen together, mostly this dense knot of gas. With the Space telescope, astrophysicists can separate the star from this very close ejecta, allowing them to piece together a clear image of the object, as in the figure above.

2014 was a unique year for Eta Car Observations. Once every 5.5 years the secondary star, which makes a long cigar shaped orbit around its primary star, moves in close to the primary star. The secondary star moves slowly most of the time, except when it gets close the primary, then it “whips around quickly.” When this close pass happens it pulls gas out of the main star. The spectrum changes for a few weeks in the observations. Mass also gets pulled out of the smaller star. When these two outflows collide in space they form a shock front. Most of the observations the team have made can be explained by the plasma physics involved in the colliding winds or shock front. They observed this event in 2003, 2009 and again this year and have found that the differences between each event have been huge. Every one of the last three events has been very different from the earlier ones with less mass loss and more ionizing radiation visible. Davidson says that one of the implications of the changes is that this event is the last one that they will be able to observe. “The gas density is so low now that the violent happenings probably may not occur anymore.”

This intensive study of the colliding winds between these two stars could be critical for astronomers understand the physics of imposter events and binary stars. Davidson likens the understanding of imposter events to the level that scientists had of supernovae in the 1950s. In the 1840s the great eruption of Eta Car was 20 solar masses (20 mass of the sun). The average supernova weighs less than 20 solar masses total. “Eta lost 20 solar masses in one explosion and most of it is still there. We don’t know the initial mass but based on its luminosity, it was originally 150 – 200 solar masses, at the upper end of the most massive stars.”

Davidson and Humphreys work is fairly unique in that it is an entirely Minnesota-based collaboration. They are the Principal Investigators and have worked with their students and post docs over the years to reach these conclusions. Among those who have worked on the project are Andrea Mehner, former U of M Ph.D. student, (now astronomer at ESO in Chile), Kazunori Ishibashi (former Ph.D. student, now at Nagoya Univ. in Japan), and John Martin of UI Springfield (former postdoc).